1. Field
This patent specification describes a method and apparatus for a high-efficiency DC stabilized power supply, and more particularly a method and apparatus for a high-efficiency DC stabilized power supply capable of reducing electrical noises and ripples by effectively controlling an output voltage of a switching regulator.
2. Discussion of the Background
Background DC (direct current) stabilized power supply apparatuses commonly used generally include two typical switching regulators, one having a high efficiency but producing a ripple on an output electric voltage and large operational noises and the other having a low efficiency producing a less ripple and low operational noises. In order to attempt to provide a DC stabilized power supply apparatus producing a less ripple, a power source voltage derived from a DC power source is applied to a switching regulator and the switching regulator generates an output voltage required as an input voltage. Then, the output voltage generated by the switching regulator is input to a series regulator and the series regulator generates a desired output voltage to be provided to a load circuit. Thus, it is possible to provide a DC stabilized power supply apparatus capable of efficiently using characteristics of the switching regulator and the series regulator to minimize a loss of electric power by the series regulator while reducing ripples.
Japanese Laid-Open Patent Application Publication, No. 07-095765, describes a DC stabilized power supply apparatus having a structure using the above-described structural concept. The structure of this DC stabilized power supply apparatus is shown in FIG. 1. In FIG. 1, reference numeral 200 denotes the DC stabilized power supply apparatus which includes a switching regulator 201 and a series regulator 202. In the switching regulator 201, an output voltage VoA is divided with resistors Ra and Rb. Then, a difference between the divided voltage and a predetermined reference voltage Vr supplied from a reference voltage generator 203 is amplified with an error amplifier Aa. After that, a voltage of an output signal from the comparator Aa and an output signal from an oscillating circuit 204 for generating a triangular pulse signal are compared by a comparator Ab. The comparator Ab controls the operation of a base current amplifying transistor Qc in accordance with the comparison result so as to control an on-time of a switching transistor Qa.
The signal output from the switching transistor Qa is smoothed through a smoothing circuit which includes a diode Da, an electric coil La, and a capacitor Ca, and becomes the output voltage VoA to be input to the series regulator 202. The output voltage VoA from the switching regulator 201 is set to be a sum voltage of the output voltage VoB from the series regulator 202 and a voltage Vce between the collector and the emitter of an output control transistor Qc of the series regulator 202 by adjusting the resistors Ra and Rb.
In the series regulator 202, the output voltage VoB is divided with resistors Rc and Rd. Then, a difference between the divided voltage and the predetermined reference voltage Vr supplied from the reference voltage generator 203 is amplified with an error amplifier Ac. The error amplifier Ac controls the operation of a base current amplifying transistor Qd to control the operation of a switching transistor Qc so that the output voltage VoB becomes constant. As shown in FIG. 1, the reference voltage generator 203 is shared by the switching regulator 201 and the series regulator 202.
In this way, the switching regulator 201 adds the output voltage VoA, including the required collector-emitter voltage Vce of the output control transistor Qc and a minimal marginal voltage, to the output voltage VoB so that the loss of power by the series regulator 202 can be minimized. This improves, at the same time, the entire efficiency of the DC stabilized power supply apparatus 200 while producing a constant voltage with less noise and ripple.
In recent years, electronic equipment has been provided with plural functions which, however, generally will not be operated at the same time and only necessary functions are activated depending upon the operational conditions of the equipment. Each of the electrical circuits of the electronic equipment performing these functions is generally designed to be applied with an optimal electric power voltage. Therefore, the electronic equipment is required to switch the electric power voltages frequently in accordance with the operational conditions. Thus, it becomes possible to make the electronic equipment environmentally friendly, and operable with a less electric power.
In addition, in a portable information handling apparatus using batteries, such as a cellular phone, a mobile personal information terminal, a laptop PC, and so forth, a reduction of power consumption is important to make the batteries last longer. Therefore, a technique for switching the electric power voltages suitably for each of the circuits becomes more and more important.
However, no particular attention to the technique for switching the electric power voltages, as described above, is invested in the structure of the DC stabilized power supply apparatus described in Japanese Laid-Open Patent Application Publication, No. 07-095765. If the output voltage, e.g., the reference voltage Vr of FIG. 1, is forcedly changed, the output voltage VoB of the series regulator 202 which is the final output voltage is greatly deviated due to a difference in response time between the switching regulator and the series regulator. As a result, a load circuit 211 connected to the series regulator 202 may be forced to break off its operation, or may suffer damage.
Furthermore, since the collector-emitter voltage Vce required for the output control transistor Qc of the series regulator 202 is changed in accordance with the output voltage of the series regulator 202 and a current amount flowing through the load circuit 211, it is needed to adjust the output voltage of the switching regulator 201 to cope with these deviation factors. However, in the circuit shown in Japanese Laid-Open Patent Application Publication, No. 07-095765, the output voltage of the switching regulator 201 is determined by the resistors Ra and Rb and cannot be changed in an arbitrary fashion.